1. Field of the Invention
The present invention relates to a sheet stacking apparatus and a sheet processing apparatus for performing a sheet stack process and an image forming apparatus, such as a copying machine and a printer, which includes the sheet processing apparatus.
2. Description of Related Art
Conventionally, in some pieces of the image forming apparatus which forms an image on a sheet, the sheet processing apparatus which makes a bundle of sheets, in which images are formed, to perform a binding process is provided as a component of the image forming apparatus.
In the sheet processing apparatus, the sheet is sequentially received by a stack tray, the sheet is aligned by abutting a sheet end on an abutment member called trailing-end fence, and the bundle of sheets is made to perform the binding process. The stack tray is disposed with an inclination angle in order to prevent the sheet processing apparatus from enlarging.
When the stack tray is disposed with an inclination angle, a self weight easily generates buckling near the abutment member provided below the inclination, which lowers alignment performance. Particularly the buckling becomes prominent in poor fragile paper or a curled sheet. Therefore, there is a device which holds a sheet end in order to prevent the buckling of the sheet.
For example, in a technique disclosed in U.S. Pat. No. 6,145,825, a regulating pressing member is disposed between the abutment member and a brush roller which abuts the sheet end on the abutment member, and the regulating pressing member presses an upper surface of the sheet in a sheet end binding process region such that the buckling of the sheet is not generated.
However, in the case where the stack tray has the large inclination angle, possibly the buckling or wave of the sheet is generated by the self weight of the sheet when the regulating pressing member is returned from a pressing position to a home position in conveying the next sheet.
Because the sheet end is regulated and pressed only near the abutment member, the buckling or wave of the sheet is generated in other regions, a conveying defect is generated in conveying the next sheet, or an alignment defect is generated by obstructing an alignment operation.
In view of the foregoing, an object of the invention is to provide a sheet stacking apparatus, a sheet processing apparatus, and an image forming apparatus, in which the buckling of the sheet is securely prevented to improve sheet conveying performance and sheet alignment performance.